In an optical communication system, an optical relay or an optical branching unit may be installed in a readily inaccessible location such as the seabed. Accordingly, it is preferable to control the optical relay or the optical branching unit remotely through transmitting a control signal from an optical terminal equipment or the like.
PTL 1 discloses a technique to transmit an optical signal obtained by superposing a monitoring control signal on a data signal in an optical communication system. In the optical communication system described in PTL 1, an optical signal is generated by superposing the monitoring control signal whose intensity is changed to be “0” or “1” on the data signal, and the generated optical signal is transmitted through an optical transmission line composed of optical fibers.